1. Field of the Invention
The invention relates to a broadband circularly polarized annular ring slot antenna and, more particularly, to a broadband circularly polarized annular ring slot antenna for increasing the 3-dB-axial-ratio bandwidth without increasing the antenna size.
2. Description of Related Art
Circularly polarized (CP) antennas, known for their capabilities of reducing polarization mismatch and moderately suppressing multipath interferences, require larger bandwidths of greater than 30% to support emerging wireless-communication applications, especially those involving satellite communications. Because of the advantages of large impedance bandwidths and low profiles, printed slot antennas have received much attention in antenna design requiring enhanced 3-dB axial-ratio bandwidths.
As shown in FIGS. 1A and 1B, the conventional broadband circularly polarized annular ring slot antenna includes: a substrate 11, an annular ring slot antenna portion 12 and a microstrip feeding portion 13, wherein the annular ring slot antenna portion 12 is located on the upper surface 111 of the substrate 11, and the microstrip feeding portion 13 is located on the lower surface 112 of the substrate 11. Besides, as shown in FIG. 1A, the annular ring slot antenna portion 12 includes a grounding unit 121, an annular ring slot unit 122, a central metal unit 123 and a coupling slot unit 124. The coupling slot unit 124 is extended from a side edge of the annular ring slot unit 122. The annular ring slot unit 122 and the coupling slot unit 124 are both surrounded by the grounding unit 121. The central metal unit 123 is surrounded by the annular ring slot unit 122.
With reference to FIG. 1A, the microstrip feeding portion 13 includes a first vertical feeding unit 131, a second vertical feeding unit 132 and a rectangular microstrip unit 133. The rectangular microstrip unit 133 is connected with the first vertical feeding unit 131 and the second vertical feeding unit 132 respectively. Besides, the first vertical feeding unit 131 is extended from the rectangular microstrip unit 133 toward a side edge 113 of the substrate 11, and the second vertical feeding unit 132 is extended from the rectangular microstrip unit 133 in the direction departing from the first vertical feeding unit 131. Moreover, the rectangular microstrip unit 133 of the microstrip feeding portion 13 corresponds to the coupling slot unit 124 of the annular ring slot antenna portion 12.
In practical measurement, the 3-dB-axial-ratio bandwidth of the conventional broadband circularly polarized annular ring slot antenna is only between 3.5% and 11%, which is far below the aforementioned standard (i.e. large than 32%). Therefore, it is desired to have a significant improvement on the circular polarization performance of the conventional circularly polarized annular ring slot antenna. However, in order to improve the 3-dB-axial-ratio bandwidth of the conventional broadband circularly polarized annular ring slot antenna, not only the size of the annular ring slot unit and the grounding unit of the conventional broadband circularly polarized annular ring slot antenna have to be enlarged, but also the slot width of the annular ring slot unit of the annular ring slot antenna portion needs to be enlarged for forming the so-called wide-slot type ring slot unit. Accordingly, the whole size of the conventional broadband circularly polarized annular ring slot antenna is inevitably to be greatly increased.